Various publications are referenced throughout this publication, and full citations for each of these publications are provided at the end of the Detailed Description.
Aeromonas salmonicida, a Gram-negative, facultatively anaerobic, non-motile, rod shaped bacterium, growing at temperatures around 20° C., is the etiological agent of furunculosis in salmonids, causing most severe economic losses in production farms of salmon and trout. The disease is characterized in the sub-acute or chronic form by the presence of haemorrhagic necrotic lesions in the gills, gut and muscle, while in the acute form fish die apparently from toxaemia without showing particular external signs.
Due to the high contagiousity of the disease and the high mortality in salmon of all ages, particularly in the sea water growers, large amounts of antibiotics are used in closed and open waters for therapy of furunculoses (Munro and Hastings, 1993). Vaccination has become an important strategy to control furunculoses in fish farms (Ellis, 1997). However, the currently applied whole cell antigen vaccines seem to show considerable variability in efficacy, the origin of which remains currently unexplained (Thornton et al., 1993).
Knowledge of the mechanisms of pathogenicity of A. salmonicida, and in particular of the main virulence factors involved, is essential in the development of efficient strategies to prevent outbreaks of furunculoses caused by A. salmonicida. Currently, several potential virulence factors of A. salmonicida have been reported, including a surface-layer protein (Chu et al., 1991), the hemolysins ASH1, ASH3, ASH4 (Hirono and Aoki, 1993), salmolysin (Titball and Munn, 1985), the serine protease AspA (Whitby et al., 1992) and the glycerolipid-cholesterol acyltransferase (GCAT) (Lee and Ellis, 1990), but their role in pathogenesis is unclear and many of them seem not to play a primary role in virulence. This was demonstrated by A. salmonicida strains with deletion mutants of the GCAT and aspA genes which had no influence on virulence of the strains in inducing furunculoses.